The present invention generally relates to organic thin-film photosensitive optoelectronic devices. More specifically, it is directed to organic photosensitive optoelectronic devices, e.g., solar cells and visible spectrum photodetectors, having an exciton blocking layer.
Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Solar cells, also known as photovoltaic (PV) devices, are specifically used to generate electrical power. PV devices are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. As used herein the term xe2x80x9cresistive loadxe2x80x9d refers to any power consuming or storing device, equipment or system.
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g. crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. Herein the term xe2x80x9csemiconductorxe2x80x9d denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term xe2x80x9cphotoconductivexe2x80x9d generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material. The terms xe2x80x9cphotoconductorxe2x80x9d and xe2x80x9cphotoconductive materialxe2x80x9d are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation of selected spectral energies to generate electric charge carriers. Solar cells are characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. Present commercially available amorphous silicon cells have stabilized efficiencies between 4 and 8%. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies with economical production costs.
PV devices typically have the property that when they are connected across a load and are irradiated by light they produce a photogenerated voltage. When irradiated without any external electronic load, a PV device generates its maximum possible voltage, V open-circuit, or Voc. If a PV device is irradiated with its electrical contacts shorted, a maximum short-circuit current, or Isc, is produced. When actually used to generate power, a PV device is connected to a finite resistive load and the power output is given by the current voltage product, Ixc3x97V. The maximum total power generated by a PV device is inherently incapable of exceeding the product, Iscxc3x97Voc. When the load value is optimized for maximum power extraction, the current and voltage have values, Imax and Vmax respectively. A figure of merit for solar cells is the fill factor ff defined as:                     ff        =                                            I              max                        ⁢                          V              max                                                          I              SC                        ⁢                          V              OC                                                          (        1        )            
where ff is always less than 1 since in actual use Isc and Voc are never obtained simultaneously. Nonetheless, as ff approaches 1, the device is more efficient.
When electromagnetic radiation of an appropriate energy is incident upon a semiconductive organic material, for example, an organic molecular crystal (OMC) material, or a polymer, a photon can be absorbed to produce an excited molecular state. This is represented symbolically as S0+hvxe2x86x92S0*. Here S0 and S0* denote ground and excited molecular states, respectively. This energy absorption is associated with the promotion of an electron from a bound state in the valence band, which may be a xcfx80-bond, to the conduction band, which may be a xcfx80*-bond, or equivalently, the promotion of a hole from the conduction band to the valence band. In organic thin-film photoconductors, the generated molecular state is generally believed to be an exciton, i.e., an electron-hole pair in a bound state which is transported as a quasi-particle. The excitons can have an appreciable life-time before geminate recombination, which refers to the process of the original electron and hole recombining with each other as opposed to recombination with holes or electrons from other pairs. To produce a photocurrent the electron-hole pair must become separated. If the charges do not separate, they can recombine in a geminant recombination process, also known as quenching, either radiativelyxe2x80x94re-emitting light of a lower than incident light energy-, or non-radiativelyxe2x80x94with the production of heat.
Either of these outcomes is undesirable in a photosensitive optoelectronic device. While exciton ionization, or dissociation, is not completely understood, it is generally believed to occur at defects, impurities, contacts, interfaces or other inhomogeneities. Frequently, the ionization occurs in the electric field induced around a crystal defect, denoted, M. This reaction is denoted S0*+Mxe2x86x92e+h+. If the ionization occurs at a random defect in a region of material without an overall electric field, the generated electron-hole pair will likely recombine. To achieve a useful photocurrent, the electron and hole must be collected separately at respective opposing electrodes, which are frequently referred to as contacts. Exciton dissociation occurs either in high electric field regions by field-emission, or at an interface between, e.g., donor-like and acceptor-like materials such as copper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI), by charge transfer. The latter can be viewed as an exothermic chemical reaction, i.e., a reaction in which some energy is released as vibrational energy. This reaction occurs because the energy separation of the dissociated exciton, i.e., the energy difference between the free electron in, e.g., PTCBI, and the free hole in, e.g., CuPc, is smaller than the energy of the exciton prior to dissociation.
Electric fields or inhomogeneities at a contact may cause an exciton to quench rather than dissociate, resulting in no net contribution to the current. Therefore, it is desirable to keep photogenerated excitons away from the contacts. This has the effect of limiting the diffusion of excitons to the region near the junction so that the junction associated electric field has an increased opportunity to separate charge carriers liberated by the dissociation of the excitons near the junction.
Here appreciation should be taken of some of the distinctions between organic photosensitive optoelectronic devices (OPODs) and organic light emitting devices (OLEDs). In an OLED, a bias is applied to a device to produce a flow of holes and electrons into a device. In OLEDs, excitons are generally formed which in time may either recombine radiatively or nonradiatively. In OLEDs, maximum radiative recombination is the desired result. In OPODs maximum exciton generation and dissociation is the desired result. The differing objectives of the devices lead to differing selection of materials and layer thicknesses. OPOD photosensitive materials are chosen for their absorption properties while photoluminescent materials for OLEDs are chosen for their emissive properties.
To produce internally generated electric fields which occupy a substantial volume, the usual method is to juxtapose two layers of material with appropriately selected conductive properties, especially with respect to their distribution of molecular quantum energy states. The interface of these two materials is called a photovoltaic heterojunction. In traditional semiconductor theory, materials for forming PV heterojunctions have been denoted as generally being of either n, or donor, type or p, or acceptor, type. Here n-type denotes that the majority carrier type is the electron. This could be viewed as the material having many electrons in relatively free energy states. The p-type denotes that the majority carrier type is the hole. Such material has many holes in relatively free energy states. The type of the background, i.e., not photogenerated, majority carrier concentration depends primarily on unintentional doping by defects or impurities. The type and concentration of impurities determine the value of the Fermi energy, or level, within the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the HOMO-LUMO gap. The Fermi energy characterizes the statistical occupation of molecular quantum energy states denoted by the value of energy for which the probability of occupation is equal to xc2xd. A Fermi energy near the LUMO energy indicates that electrons are the predominant carrier. A Fermi energy near the HOMO energy indicates that holes are the predominant carrier. Accordingly, the Fermi energy is a primary characterizing property of traditional semiconductors and the prototypical PV heterojunction has traditionally been the p-n interface.
In addition to relative free-carrier concentrations, a significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. As opposed to free carrier concentrations, carrier mobility is determined in large part by intrinsic properties of the organic material such as crystal symmetry and periodicity. Appropriate symmetry and periodicity can produce higher quantum wavefunction overlap of HOMO levels producing higher hole mobility, or similarly, higher overlap of LUMO levels to produce higher electron mobility. Moreover, the donor or acceptor nature of an organic semiconductor, e.g., 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), may be at odds with the higher carrier mobility. For example, while chemistry arguments suggest a donor, or n-type, character for PTCDA, experiments indicate that hole mobilities exceed electron mobilities by several orders of magnitude so that the hole mobility is a critical factor. The result is that device configuration predictions from donor/acceptor criteria may not be borne out by actual device performance. Due to these unique electronic properties of organic materials, rather than designating them as xe2x80x9cp-typexe2x80x9d or xe2x80x9cacceptor-typexe2x80x9d and xe2x80x9cn-typexe2x80x9d or xe2x80x9cdonor-typexe2x80x9d, the nomenclature of xe2x80x9chole-transporting-layerxe2x80x9d (HTL) or xe2x80x9celectron-transporting-layerxe2x80x9d (ETL) is frequently used. In this designation scheme, an ETL will preferentially be electron conducting and an HTL will preferentially be hole transporting. The term xe2x80x9crectifyingxe2x80x9d denotes, inter alia, that an interface has an asymmetric conduction characteristic, i.e., the interface supports electronic charge transport preferably in one direction. Rectification is associated normally with a built-in electric field which occurs at the heterojunction between appropriately selected materials.
The electrodes, or contacts, used in a photosensitive optoelectronic device are an important consideration. In a photosensitive optoelectronic device, it is desirable to allow the maximum amount of ambient electromagnetic radiation from the device exterior to be admitted to the photoconductively active interior region. That is, it is desirable to get the electromagnetic radiation to where it can be converted to electricity by photoconductive absorption. This often dictates that at least one of the electrical contacts should be minimally absorbing and minimally reflecting of the incident electromagnetic radiation. That is, such contact should be substantially transparent. When used herein, the terms xe2x80x9celectrodexe2x80x9d and xe2x80x9ccontactxe2x80x9d refer only to layers that provide a medium for delivering photogenerated power to an external circuit or providing a bias voltage to the device. That is, an electrode, or contact, provides the interface between the photoconductively active regions of an organic photosensitive optoelectronic device and a wire, lead, trace or other means for transporting the charge carriers to or from the external circuit. The term xe2x80x9ccharge transfer layerxe2x80x9d is used herein to refer to layers similar to but different from electrodes in that a charge transfer layer only delivers charge carriers from one subsection of an optoelectronic device to the adjacent subsection. As used herein, a layer of material or a sequence of several layers of different materials is said to be xe2x80x9ctransparentxe2x80x9d when the layer or layers permit at least 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through the layer or layers. Similarly, layers which permit some but less that 50% transmission of ambient electromagnetic radiation in relevant wavelengths are said to be xe2x80x9csemi-transparentxe2x80x9d.
Electrodes or contacts are usually metals or xe2x80x9cmetal substitutesxe2x80x9d. Herein the term xe2x80x9cmetalxe2x80x9d is used to embrace both materials composed of an elementally pure metal, e.g., Mg, and also metal alloys which are materials composed of two or more elementally pure metals, e.g., Mg and Ag together, denoted Mg:Ag. Here, the term xe2x80x9cmetal substitutexe2x80x9d refers to a material that is not a metal within the normal definition, but which has the metal-like properties that are desired in certain appropriate applications. Commonly used metal substitutes for electrodes and charge transfer layers would include doped wide bandgap semiconductors, for example, transparent conducting oxides such as indium tin oxide (ITO), gallium indium tin oxide (GITO), and zinc indium tin oxide (ZITO). In particular, ITO is a highly doped degenerate n+ semiconductor with an optical bandgap of approximately 3.2 eV rendering it transparent to wavelengths greater than approximately 3900 xc3x85. Another suitable metal substitute material is the transparent conductive polymer polyanaline (PANI) and its chemical relatives. Metal substitutes may be further selected from a wide range of non-metallic materials, wherein the term xe2x80x9cnon-metallicxe2x80x9d is meant to embrace a wide range of materials provided that the material is free of metal in its chemically uncombined form. When a metal is present in its chemically uncombined form, either alone or in combination with one or more other metals as an alloy, the metal may alternatively be referred to as being present in its metallic form or as being a xe2x80x9cfree metalxe2x80x9d. Thus, the metal substitute electrodes of the present invention may sometimes be referred to as xe2x80x9cmetal-freexe2x80x9d wherein the term xe2x80x9cmetal-freexe2x80x9d is expressly meant to embrace a material free of metal in its chemically uncombined form. Free metals typically have a form of metallic bonding that may be thought of as a type of chemical bonding that results from a sea of valence electrons which are free to move in an electronic conduction band throughout the metal lattice. While metal substitutes may contain metal constituents they are xe2x80x9cnon-metallicxe2x80x9d on several bases. They are not pure free-metals nor are they alloys of free-metals. When metals are present in their metallic form, the electronic conduction band tends to provide, among other metallic properties, a high electrical conductivity as well as a high reflectivity for optical radiation.
A typical prior art photovoltaic device configuration is the organic bilayer cell. In the bilayer cell, charge separation predominantly occurs at the organic heterojunction. The built-in potential is determined by the HOMO-LUMO gap energy difference between the two materials contacting to form the heterojunction. The HOMO-LUMO energy levels for such a heterojunction are illustrated schematically in FIG. 1 where 101 represents an anode, 102 represents an HTL layer, 103 represents an ETL layer and 104 represents a cathode. The HOMO-LUMO gap offset between the HTL and ETL produce an electric field around the HTL/ETL interface.
Herein, the term xe2x80x9ccathodexe2x80x9d is used in the following manner. In a non-stacked PV device or a single unit of a stacked PV device under ambient irradiation and connected with a resistive load and with no externally applied voltage, e.g., a solar cell, electrons move to the cathode from the adjacent photoconducting material. Similarly, the term xe2x80x9canodexe2x80x9d is used herein such that in a solar cell under illumination, holes move to the anode from the adjacent photoconducting material, which is equivalent to electrons moving in the opposite manner. It will be noted that as the terms are used herein anodes and cathodes may be electrodes or charge transfer layers.
Organic PV devices typically have relatively low quantum yield (the ratio of photons absorbed to carrier pairs generated, or electromagnetic radiation to electricity conversion efficiency), being on the order of 1% or less. This is, in part, thought to be due to the second order nature of the intrinsic photoconductive process, that is, carrier generation requires exciton generation, diffusion and ionization, as described above. In order to increase these yields, materials and device configurations are desirable which can enhance the quantum yield and, therefore, the power conversion efficiency.
Thompson et al. in U.S. patent application Ser. No. 09/311,126, now abandoned, for xe2x80x9cVery High Efficiency Organic Light Emitting Devices Based on Electrophosphorescencexe2x80x9d have described the use of an exciton blocking layer to confine excitons to the emission layer in an organic light emitting device (OLED) in order to increase the device efficiency. In the context of the present invention, an EBL is characterized by its ability to prevent the diffusion of excitons from an adjacent organic layer into or across the EBL. xe2x80x9cUltrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniquesxe2x80x9d, Chemical Reviews, Vol. 97, No. 6, 1997 (hereinafter Forrest, Chem. Rev. 1997) and Arbour, C., Armstrong, N. R., Brina, R., Collins, G., Danziger, J. -P., Lee, P., Nebesny, K. W., Pankow, J., Waite, S., xe2x80x9cSurface Chemistries and Photoelectrochemistries of Thin Film Molecular Semiconductor Materialsxe2x80x9d, Molecular Crystals and Liquid Crystals, 1990, 183, 307, (hereinafter Arbour et al.), disclose that alternating thin multilayer stacks of similar type photoconductors could be used to enhance photogenerated carrier collection efficiency over that using a single layer structure. Further, these sources describe multiple quantum well (MQW) structures in which quantum size effects occur when the layer thicknesses become comparable to the exciton dimensions.
Several guidelines must be kept in mind in designing an efficient organic photosensitive optoelectronic device. It is desirable for the exciton diffusion length, LD, to be greater than or comparable to the layer thickness, L, since it is believed that most exciton dissociation will occur at an interface. If LD is less than L, then many excitons may recombine before dissociation. It is further desirable for the total photoconductive material thickness to be of the order of the electromagnetic radiation absorption length, 1/xcex1 (where xcex1 is the absorption coefficient), so that nearly all of the radiation incident on the solar cell is absorbed to produce excitons. However, the thickness should not be so large compared to the extent of the heterojunction electric fields that many excitons get generated in a field-free region. One reason for this is that the fields help to dissociate the excitons. Another reason is that if an exciton dissociates in a field-free region, it is more likely to suffer geminant recombination, or quenching, and contribute nothing to the photocurrent. Further, electric fields may exist at the electrode/semiconductor interfaces. These fields at the electrode interfaces can also promote exciton quenching. Furthermore, the photoconductive layer thickness should be as thin as possible to avoid excess series resistance due to the high bulk resistivity of organic semiconductors.
On the other hand, another countervailing consideration is that as the separation between the exciton dissociating interface and the adjacent electrodes increases, the electric field region around the interface will have a higher value over a greater volume. Since light absorption increases with increasing electric field strength, more excitons will be generated. Also, the higher electric fields will also promote faster exciton dissociation.
It has been suggested that one means for circumventing the diffusion length limitation is to use thin cells with multiple or highly folded interfaces, such as can be achieved using nanotextured materials, polymer blends, closely spaced, repeated interfaces, or spatially distributed dissociation sites. To date, none of these proposals has led to a significant improvement in overall performance of solar cells, particularly at high illumination intensities.
Accordingly, in the present invention higher internal and external quantum efficiencies have been achieved by the inclusion in OPODs of one or more exciton blocking layers (EBLs) to confine photogenerated excitons to the region near the dissociating interface and prevent parasitic exciton quenching at a photosensitive organic/electrode interface. In addition to limiting the volume over which excitons may diffuse, an EBL can also act as a diffusion barrier to substances introduced during deposition of the electrodes. In some circumstances, an EBL can be made thick enough to fill pinholes or shorting defects which could otherwise render an OPOD non-functional. An exciton blocking layer can therefore help protect fragile organic layers from damage produced when electrodes are deposited onto the organic materials.
It is believed that the EBLs comprising the present invention derive their exciton blocking property from having a LUMO-HOMO energy gap substantially larger than that of the adjacent organic semiconductor from which excitons are being blocked. The thus confined excitons are prohibited from existing in the EBL due to quantum energy considerations. While it is desirable for the EBL to block excitons, it is not desirable for the EBL to block all charge carrying quanta as well. However, due to the nature of the adjacent energy levels an EBL will necessarily block one sign of charge carrier. By design, an EBL will always exist between two adjacent layers, usually an organic photosensitive semiconductor layer and a electrode or charge transfer layer. The adjacent electrode or charge transfer layer will be in context either a cathode or an anode. Therefore, the material for an EBL in a given position in a device will be chosen so that the desired sign of carrier will not be impeded in its transport to the electrode or charge transfer layer. Proper energy level alignment ensures that no barrier to charge transport exists, preventing an increase in series resistance. It should be appreciated that the exciton blocking nature of a material is not an intrinsic property. Whether a given material will act as an exciton blocker depends upon the relative HOMO and LUMO levels of the adjacent organic photosensitive material. Therefore, it is not possible to identify a class of compounds in isolation as exciton blockers without regard to the device context in which they may be used. However, with the teachings herein one of ordinary skill in the art may identify whether a given material will function as an exciton blocking layer when used with a selected set of materials to construct an OPOD.
For example, FIGS. 2A through 2C illustrate three types of bilayer OPODs having one or more EBLs to suppress undesired exciton diffusion and enhance device efficiency. These figures schematically depict the relative energy levels of the various materials comprising various embodiments of an OPOD cell having one or more EBLs. The lines in each figure at the ends represent the work function of the electrodes or charge transfer layers at the ends. The shaded boxes represent the relative LUMO-HOMO energy gaps of the various constituent layers of the OPOD.
With regard to FIG. 2A, OPOD device 2A00 comprises an anode layer 2A01, such as indium tin oxide (ITO), a hole transporting layer (HTL) 2A02, such as CuPc which is believed to have a LUMO-HOMO separation of approximately 1.7 eV, an electron transporting layer (ETL) 2A03, such as PTCBI which is also believed to have a LUMO-HOMO separation of approximately 1.7 eV, an exciton blocking layer (EBL) 2A04, such as 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also called bathocuproine or BCP) which is believed to have a LUMO-HOMO separation of about 3.5 eV, and a cathode layer 2A05, such as silver. It should be appreciated that the larger LUMO-HOMO energy gap in EBL 2A04 will prohibit diffusion of excitons from ETL 2A03 into EBL 2A04. Also, coincidentally EBL 2A04 would block the transit of holes from ETL 2A03 toward the cathode due to the unfavorable gap between the HOMO levels of ETL 2A03 and EBL 2A04, i.e., the higher ionization potential of the EBL. However, this effect is thought to be of little consequence since the internal electric field generated around the HTL/ETL interface will tend to drive holes towards anode layer 1 so that there are relatively few holes near the ETL/EBL interfacial region. One result of this hole blocking aspect is that EBL 2A04 is optimally a cathode side EBL. Note also that there is incidentally a slightly unfavorable LUMO gap for electrons at the ETL/EBL interface in the illustrated example using PTCBI as the ETL and BCP as the EBL. Optimally, it is desirable for a material used as a cathode side EBL to have a LUMO level closely matching the LUMO level of the adjacent ETL material so that the undesired barrier to electrons is minimized.
With regard to FIG. 2B, the analogous situation of an anode side EBL is depicted. OPOD device 2B00 comprises an anode layer 2B01, such as indium tin oxide (ITO), an exciton blocking layer (EBL) 2B02, such as 4,4xe2x80x2,4xe2x80x3-tris{N,-(3-methylphenyl)-N-phenylamino}triphenylamine (m-MTDATA) or polyethylene dioxythiophene (PEDOT). The LUMO-HOMO separations for m-MTDATA and PEDOT are not precisely known but believed to be such as depicted in FIG. 2B. The OPOD further comprises a hole transporting layer (HTL) 2B03, such as CuPc, an electron transporting layer (ETL) 2B04, such as PTCBI, and a cathode layer 2B05, such as silver. It should be appreciated that the larger LUMO-HOMO energy gap in EBL 2B02 will prohibit diffusion of excitons from HTL 2B03 into EBL 2B02. Also, coincidentally EBL 2B02 would block the transit of electrons from HTL 2B03 toward the cathode due to the unfavorable gap between the LUMO levels of HTL 2B03 and EBL 2B02, i.e., the higher LUMO level of the EBL. However, this effect is thought to be of little consequence since the internal electric field generated around the HTL/ETL interface will tend to drive electrons towards cathode layer 2B05 so that there are relatively few electrons near the HTL/EBL interfacial region. One result of this electron blocking aspect is that EBL 2B02 is optimally an anode side EBL.
Finally, in FIG. 2C, the various relative energy layers of an OPOD 2C00 having both anode side and cathode side EBLs is illustrated. An anode layer 2C01, such as ITO, an anode side EBL 2C02, such as m-MTDATA or PEDOT, a HTL 2C03, such as CuPc, an ETL 2C04, such as PTCBI, a cathode side EBL 2C05, such as BCP, and a cathode layer 2C06, such as silver. Accordingly, with both an anode side EBL and a cathode side EBL, excitons generated within HTL 2C03 and ETL 2C04 are effectively confined until they preferably dissociate or undesirably quench.
A multilayer structure like that whose energy state structure is depicted in FIG. 2D is a highly efficient photodetector. In FIG. 2D, 2D01 is a transparent anode, e.g., ITO, which is adjacent to one of several HTL, e.g., CuPc, layers 2D02. Between the HTL layers 2D02 and adjacent to exciton blocking layer 2D04 are several ETL, e.g., PTCBI, layers 2D03. Exciton blocking layer 2D04 is BCP in this example. Exciton blocking layer 2D04 is adjacent to cathode 2D05 which is, e.g., silver. Arbour et al and Forrest, Chem. Rev. 1997 suggested that the numerous HTL-ETL interfaces can provide efficient free carrier generation when a bias is provided to extract the carriers from the device. Arbour and Forrest did not, however, suggest the use of an exciton blocking layer as described herein to further enhance the efficiency in such devices.
OPODs operating without a bias and including an EBL in accordance with the present invention can be made very thin without severe loss of photocurrent. Accordingly, OPODs including EBLs may be used in combination with the highly efficient OPODs of the U.S. patent applications of Forrest et al., now Pat. Nos. 6,352,777, 6,297,495, 6,278,055, 6,198,092 and 6,198,091, (hereinafter collectively xe2x80x9cForrest OPOD Appls.xe2x80x9d) which are incorporated herein by reference in their entirety. Stacked OPODs including EBLs and having numerous subcells and/or including a waveguide configuration may be constructed in accord with the present invention to achieve high internal and external quantum efficiencies.
When the term xe2x80x9csubcellxe2x80x9d is used hereafter, it refers to an organic photosensitive optoelectronic construction which may include an exciton blocking layer in accordance with the present invention. When a subcell is used individually as a photosensitive optoelectronic device, it typically includes a complete set of electrodes, i.e., positive and negative. As disclosed herein, in some stacked configurations it is possible for adjacent subcells to utilize common, i.e., shared, electrode or charge transfer layers. In other cases, adjacent subcells do not share common electrodes or charge transfer layers. The term xe2x80x9csubcellxe2x80x9d is disclosed herein to encompass the subunit construction regardless of whether each subunit has its own distinct electrodes or shares electrodes or charge transfer layers with adjacent subunits. Herein the terms xe2x80x9ccellxe2x80x9d, xe2x80x9csubcellxe2x80x9d, xe2x80x9cunitxe2x80x9d, xe2x80x9csubunitxe2x80x9d, xe2x80x9csectionxe2x80x9d, and xe2x80x9csubsectionxe2x80x9d are used interchangeably to refer to a photoconductive layer or set of layers and the adjoining electrodes or charge transfer layers. As used herein, the terms xe2x80x9cstackxe2x80x9d, xe2x80x9cstackedxe2x80x9d, xe2x80x9cmultisectionxe2x80x9d and xe2x80x9cmulticellxe2x80x9d refer to any optoelectronic device with multiple layers of a photoconductive material separated by one or more electrode or charge transfer layers.
Since the stacked subcells of the solar cell may be fabricated using vacuum deposition techniques that allow external electrical connections to be made to the electrodes separating the subcells, each of the subcells in the device may be electrically connected either in parallel or in series, depending on whether the power and/or voltage generated by the solar cell is to be maximized. The improved external quantum efficiency that may be achieved for stacked solar cell embodiments of the present invention may also be attributed to the fact that the subcells of the stacked solar cell may be electrically connected in parallel since a parallel electrical configuration permits substantially higher fill factors to be realized than when the subcells are connected in series.
Although the high series resistance of photoconductive organic materials inhibits use of subcells in a series configuration for high power applications, there are certain applications, for example, in operating liquid crystal displays (LCD), for which a higher voltage may be required, but only at low current and, thus, at low power levels. For this type of application, stacked, series-connected solar cells may be suitable for providing the required voltage to the LCD. In the case when the solar cell is comprised of subcells electrically connected in series so as to produce such a higher voltage device, the stacked solar cell may be fabricated so as to have each subcell producing approximately the same current so to reduce inefficiency. For example, if the incident radiation passes through in only one direction, the stacked subcells may have an increasing thickness with the outermost subcell, which is most directly exposed to the incident radiation, being the thinnest. Alternatively, if the subcells are superposed on a reflective surface, the thicknesses of the individual subcells may be adjusted to account for the total combined radiation admitted to each subcell from the original and reflected directions.
Further, it may be desirable to have a direct current power supply capable of producing a number of different voltages. For this application, external connections to intervening electrodes could have great utility. Accordingly, in addition to being capable of providing the maximum voltage that is generated across the entire set of subcells, an exemplary embodiment the stacked solar cells of the present invention may also be used to provide multiple voltages from a single power source by tapping a selected voltage from a selected subset of subcells.
Representative embodiments may also comprise transparent charge transfer layers. As described herein charge transfer layers are distinguished from ETL and HTL layers by the fact that charge transfer layers are frequently, but not necessarily, inorganic and they are generally chosen not to be photoconductively active.
Embodiments of the present invention may include, as one or more of the transparent electrodes of the optoelectronic device, a highly transparent, non-metallic, low resistance cathode such as disclosed in U.S. patent application Ser. No. 09/054,707 to Parthasarathy et al. (xe2x80x9cParasarathy ""707xe2x80x9d) , or a highly efficient, low resistance metallic/non-metallic composite cathode such as disclosed in U.S. Pat. No. 5,703,436 to Forrest et al. (xe2x80x9cForrest ""436xe2x80x9d). Each type of cathode is preferably prepared in a fabrication process that includes the step of sputter depositing an ITO layer onto either an organic material, such as copper phthalocyanine (CuPc), PTCDA and PTCBI, to form a highly transparent, non-metallic, low resistance cathode or onto a thin Mg:Ag layer to form a highly efficient, low resistance metallic/non-metallic composite cathode. Parasarathy ""707 discloses that an ITO layer onto which an organic layer had been deposited, instead of an organic layer onto which the ITO layer had been deposited, does not function as an efficient cathode.
It is an object of the present invention to provide an OPOD and an OPOD subcell comprising one or more exciton blocking layers to increase the internal quantum efficiency of the OPOD or OPOD subcell.
It is an object of the present invention to provide an OPOD capable of operating with a high external quantum efficiency and comprising stacked OPOD subcells.
It is another object of the present invention to provide a stacked OPOD capable of operating with an external quantum efficiency that approaches the maximum internal quantum efficiency of an optimal OPOD subcell.
Another object of the present invention is to provide an OPOD with improved absorption of incident radiation for more efficient photogeneration of charge carriers.
It is a further objective of the present invention to provide an OPOD with an improved Voc and an improved Isc.
Another object of the present invention is to provide a stacked OPOD having parallel electrical interconnection of the subcells.
A further object of the present invention is to provide a stacked OPOD comprised of multiple organic OPOD subcells with transparent electrodes and having a substantially reflective bottom layer to increase overall electromagnetic radiation absorption by capturing the electromagnetic radiation reflected by the bottom layer.
A further object of the present invention is to provide a waveguide configuration OPOD having an exciton blocking layer.
Yet another object of the present invention is to provide OPODs including a conductive or an insulating substrate.
A further object of the present invention is to provide OPODs including a rigid or a flexible substrate.
A further object of the present invention is to provide OPODs wherein the organic materials used are polymeric or non-polymeric thin films.